The present invention relates to a liquid fuel burner of the type having a porous member consisting of a fuel receiving section arranged for receiving a liquid fuel supplied to the burner and an evaporation section in which the liquid fuel coming from the fuel receiving section is evaporated.
Hitherto, many types of liquid fuel burners designed and constructed to perform burning at a constant rate have been used, and various attempts and proposals have been made to maintain such burners at a constant burning rate under varying environmental condition. Namely, in the liquid fuel burner, there is a problem that a fluctuation of the environmental condition causes a change in the burning rate which in turn changes the air-fuel ratio and the temperature of the mixture gas consisting of air and vaporized fuel, resulting in a large fluctuation of the burning characteristic even when the same fuel is used. Thus, it is quite difficult to obtain a so-called clean burning condition in which the mixture is burnt stably at a high quality of burning. The efforts for maintaining a constant burning rate have been attributed to this fact.
The above-mentioned problem occurs also in liquid fuel burners in which the burning rate is controllable. Therefore, the practically usable control range of the burning rate has been restricted inconveniently to a narrow region of, for example, between 6/10 and 1/1.
On the other hand, there is an increasing demand for a wider control range of the burning rate in liquid burners. In addition, the environmental condition is inevitably varied during ordinary use of a liquid fuel burner. Under this circumstance, the conventional liquid fuel burner in which the burning rate is fixed or controllable only within a limited range cannot sufficiently meet the requirement.
In most existing liquid fuel burners of the controllable burning rate type, control of the burning rate is obtained by adjusting the flow rate of the fuel. In such burners, the fuel tends to leak from the adjusting device for adjusting the flow rate of the fuel. The fuel leaking outside the burner causes a bad smell and, in the worst case, may catch fire causing a serious accident. In addition, there is a tendency for the mechanical parts of the adjusting device to be corroded by water or organic acid contained in the fuel. Further, the adjusting device is liable to become inoperative due to sticking caused by tar-like sticky substances or sediments depositing in the adjusting device. Once such inconveniences occur, a safe condition can be restored only through troublesome and time consuming repair work, because it is necessary to disassemble the fuel system. This repair work inevitably causes contamination of the worker, burner and associated instruments as well as the area around the burner. In addition, the assembling has to be done with the greatest care in order to avoid leakage of the fuel.
Recently, a liquid fuel burner has been developed which has a fuel evaporation section incorporating a porous member arranged to be supplied with air, the rate of air supply being controllable. This burner, however, has the following disadvantage.
In this type of the burner, the vapor pressure around the evaporation section is changed in accordance with the change in the flow rate of air, so that the fuel is evaporated from the evaporation section at a rate corresponding to the flow rate of air. It is therefore possible to maintain a constant air-fuel ratio of the mixture irrespective of the rate of burning. However, the change of vapor pressure alone cannot effect efficient evaporation of fuel and, accordingly, it is impossible to obtain a large rate of burning with a large amount of fuel.
To overcome this problem, it has been proposed to form a flame in the mixing chamber and to transmit the heat of the flame to the evaporation section thereby promoting evaporation of the liquid fuel. Unfortunately, this attempt has not been particularly successful because a relatively long time elapses before the burning is stabilized. This is due to the fact that the flame does not grow rapidly after ignition thereby producing a certain time lag before evaporation of the fuel. In addition, when the burner is turned off, the flame does not go out immediately so that evaporation of the fuel is continued thereby causing a lag in extinction of the fire. Further, the flame does not change smoothly when the burning rate is changed so that the burning rate cannot be changed linearly.